What are the hedge knight, the sworn sword, and the mystery knight? Were these prequels he wrote?

short story prequels starring dunk and egg. dunk is a hedge knight who ends up being the lord commander of the king's guard (duncan the tall) and egg is a targaryan prince who ends up as the unlikely king since the heirs ahead of him died or abdicated (aegon, maestre aemon's younger brother) .

they both died due to the tragedy at summerhall.
it takes place a few generations before the current stories.